1. Field of the Invention
The present invention relates to a position transducer which detects an amount of travel and moved position of a moving or movable part of machine tools, industrial robots, etc.
2. Description of the Related Art
As a position transducer which detects an amount of travel and moved position of a moving part of machines tool, industrial robots, etc., there has been proposed a position transducer 100 of a differential transformer type as shown in FIG. 1.
As shown in FIG. 1, the differential transformer type position transducer 100 includes a detection unit 110 consisting of an exciting coil 111 excited with a signal of 2 to 3 kHz in frequency for example, a first detection coil 112 disposed adjacently to one end of the exciting coil 111 and concentrically with the exciting coil 111, and a second detection coil 113 disposed adjacently to the other end of the exciting coil 111 and concentrically with the exciting coil 111, and a round bar-shaped magnetic core 122 installed with a spindle 121 to a moving part 120 of a machine tool, industrial robot or the like movably inside the detection unit 110 along the center axis of the latter as the moving part 120 moves.
The first and second detection coils 112 and 113 are longer than the moving distance of the core 122, and the exciting coil 111 is substantially as long as the first and second detection coils 112 and 113. The first and second detection coils 112 and 113 are magnetically coupled to the exciting coil 111 and given magnetic fluxes by the exciting coil 111 to induce an electric power.
In the position transducer 100 constructed as in the above, when the penetration or amount of insertion of the core 122 in the first and second detection coils 112 and 113 changes as the core 122 moves inside the detector 110, the inductance of the first and second detection coils 112 and 113 will be changed correspondingly to the amount of insertion of the core 122. Thus, the electric power from the first and second detection coils 112 and 113, induced due to the magnetic fluxes from the exciting coil 111, will change correspondingly to an amount of travel of the core 122.
Therefore, by determining a difference in output between the first and second detection coils 112 and 113, the differential transformer the position transducer 100 can detect, with a high accuracy, an amount of travel and moved position of the core 122, that is, of a moving part 120 to which the core 122 is fixed.
FIG. 2 shows an output characteristic of the above differential transformer type position transducer 100. On the assumption that a position where the penetration of the core 122 in the first detection coil 112 and that of the penetration of the core 122 in the second detection coil 113 are equal to each other is a reference position, when the core 122 is at the reference position, the inductance of the first detection coil 112 is equal to that of the second detection coil 113. Thus, an output C1 from the first detection coil 112 is equal to an output C2 from the second detection coil 113 and the difference between these outputs (C2xe2x88x92C1) is zero.
As the core 122 moves from the reference position, the amount of insertion of the core 122 in one of the detection coils increases while that of the core 122 in the other decreases. Thus the detection coil in which the amount of insertion of the core 122 is increased will have the inductance thereof linearly increased for the increased amount of insertion or amount of travel of the core 122, while the detection coil in which the amount of insertion of the core 122 is decreased will have the inductance thereof linearly decreased for the decreased amount of insertion or amount of travel of the core 122. Therefore, by converting the change in inductance of each detection coil to determine a difference in output (C2xe2x88x92C1) between the detection coils, it is possible to detect, with a high accuracy, the amount of travel and moved position of the core 122, that is, of the moving part 120 to which the core 122 is fixed, while canceling the influence of electric noises.
There has also been proposed an MI effect type position transducer having a so-called magnetic impedance effect (will be referred to as xe2x80x9cMI effectxe2x80x9d hereunder) applied therein, as a position transducer similar to the aforementioned differential transformer type position transducer 100.
The MI effect type position transducer is not illustrated herein. It includes a pair of detection coils corresponding to the first and second detection coils 112 and 113, respectively, in the aforementioned differential transformer type position transducer 100 and which is excited directly. Because of this direct excitation, this position transducer has not any coil corresponding to the exciting coil 111 in the differential transformer type position transducer 100.
In the MI effect type position transducer, when the pair of detection coils is driven with a high frequency pulse whose rate is a few MHz to tens MHz and width is a few ns to tens ns, or a sinusoidal wave having a similar pulse rate, the inductance component of the detection coils and the skin effect of a magnetic substance, developed by a high frequency excitation, will cause the actual resistance component of the detection coils to change correspondingly to a relative position of the core to the pair of detection coils. Therefore, by adapting the MI effect type position transducer to detect changes in impedance of the detection coils, which is a combination of the changes in inductance component and actual resistance component of the detection coils, it is possible to have a very wide output dynamic range and detect, with a higher accuracy, the amount of travel and moving distance of the core, namely, a moving pat to which the core is fixed.
Disadvantageously, however, the aforementioned conventional different transformer type and MI effect type position transducers as a whole can hardly be designed compact. More specifically, the conventional differential transformer type position transducer needs three coils longer than the moving distance of the core and the whole apparatus has to be more than three times longer than the moving distance (effective length for detection) of the core.
In the conventional MI effect type position transducer, there is not provided any coil corresponding to the exciting coil used in the differential transformer type position transducer. Since the magnetic field at the ends of the detection coils is less uniform and so the output linearity is lower, the length of the apparatus in the moving direction of the core has to be sufficiently longer than the effective length for detection in order to assure a high accuracy of detection. Thus, the conventional MI effect type position transducer as a whole cannot be designed so much compact as compared with the differential transformer type position transducer.
To provide a solution to the above problems of the prior art, the Inventor of the present invention has proposed a position transducer disclosed in the Japanese Published Unexamined Application No. 2000-9412. The position transducer includes first and second detection coils excited with a high frequency, and a core fixed with a spindle to a moving part of a machine tool, industrial robot or the like and moved inside the first detection coil. When the core is in place (at a reference or home position) inside the first detection coil, the impedance of the first detection coil is equal to that of the second detection coil. In this position transducer, when the core moves from the reference position, the impedance of the first detection coil changes linearly correspondingly to the amount of travel of the core while the impedance of the second detection coil is always set constant independently of the amount of travel of the core. Therefore, with this position transducer, by converting a change in impedance of the first detection coil and determining a difference in output between the first and second detection coils, it is possible to detect, with a high accuracy, the amount of travel of the core, that is, the amount of travel and moved position of the moving part fixed to the core while canceling the influence of electric noises.
In this position transducer, the first detection coil has to be longer than the amount of travel of the core (effective length for detection) while the second detection coil can be reduced in length. Therefore, the position transducer can be designed more compact on the whole.
Similarly to the aforementioned MI effect type position transducer, this position transducer shows a reduced uniformity of the magnetic field at the end of the first detection coil. To solve this problem, a magnetic material such as Permalloy having a high permeability is disposed near the end of the first detection coil to make ideally parallel the magnetic fluxes at the end of the first excitation coil, thereby reducing the magnetic field non-linearity at the end of the first detection coil.
As in the above, in the position transducer disclosed in the Japanese Published Unexamined Application No. 2000-9412, since the length of one of the pair of the detection coils is short, and the magnetic material having a high permeability is disposed near the end of the detection coil having a change in impedance to improve the magnetic field non-linearity at the end of the detection coil, the position transducer can be designed more compact on the whole by making the length of the coil of the direction in which the core moves shorter than the effective length for detection.
In the position transducer disclosed in the Japanese Published Unexamined Application No. 2000-9412, the whole apparatus is designed compact and can detect a position with a high accuracy. However, since the magnetic material having a high permeability such as Permalloy is disposed near the end of the first detection coil to reduce the magnetic field non-linearity at the end of the first detection coil, the manufacturing cost is increased and apparatus construction is complicated. Also, since the position transducer has to be assembled with a relatively high precision, the manufacturing cost is further increased.
It is therefore an object of the present invention to overcome the above-mentioned drawbacks of the prior art by providing a position transducer which assures an effective length for detection effectively with a simple construction, is designed compact on the whole with no increase of the manufacturing cost and can detect a position with a high accuracy.
The above object can be attained by providing a position transducer including a detection coil excited with a high frequency, a magnetic core inserted in the detection coil movably in relation to the latter and along the center axis of the latter, an auxiliary coil lap-wound on an end of the detection coil and driven in phase with the detection coil, and means for detecting a relative position of the core to the detection coil on the basis of an impedance change of the detection coil.
In the above position transducer, as the core is moved inside the detection coils excited with a high frequency in relation to the latter, the changes in impedance of the detection coils vary correspondingly to the amount of travel of the core. Based on the change in impedance of the detection coils, the position detecting means will detect a relative position of the core to the detection coils. Therefore, by fixing the core of the position transducer or one of the detection coils to a moving part of a machine tool, industrial robot or the like, it is possible to detect an amount of travel and moved position of the moving part.
In the above position transducer, the auxiliary coil lap-wound on the detection coil at an end of the latter and driven in phase with the detection coil reduces the magnetic field non-linearity at the end of the detection coil, whereby the position transducer can assure an effective length for detection effectively and be designed compact on the whole.
In the position transducer according to the present invention, the detection coil may include first and second detection coils disposed concentrically with and adjacent to each other, and the position detecting means may detect a relative position of the core to the detection coils is detected by determining a difference in output between the first and second detection coils.
Thus, by detecting the relative position of the core to the detection coils on the basis of the difference in output between the first and second detection coils, it is possible to cancel the influence of electric noises etc. and thus detect, with a higher accuracy, the relative position of the core to the detection coils.
Also the position transducer according to the present invention may be designed such that when the core is at a reference position inside the detection coil, the impedance of the auxiliary coil is equal to that of the detection coil and the position detecting means determines a difference in output between the detection coil and auxiliary coil to detect a relative position of the core to the detection coil.
Thus, by detecting the relative position of the core to the detection coils on the basis of the difference in output between the first and second detection coils, it is possible to cancel the influence of electric noises etc. and thus detect, with a higher accuracy, the relative position of the core to the detection coils.
Also in the position transducer according to the present invention, the auxiliary coil may include a first auxiliary coil provided at one end of the detection coil and a second auxiliary coil connected in series to the first auxiliary coil and provided at the other end of the detection coil, and when the core is at a reference position inside the detection coil, the total impedance of the first and second auxiliary coils is equal to the impedance of the detection coil and the position detecting means determines a difference in output between the detection coil and first and second auxiliary coils to detect a relative position of the core to the detection coil.
Thus, by providing the fist auxiliary coil at one end of the detection coil and the second auxiliary coil at the other end of the detection coil, it is possible to improve the magnetic field uniformity at the opposite ends of the detection coil and thus assure an effective length for detection effectively.
In the position transducer according to the present invention, there are lap-wound on the end portions of the detection coil auxiliary the auxiliary coils which are driven in phase with the detection coil to substantially increase the winding density at the end portions of the detection coil, thereby reducing the non-linearity of the magnetic field at the ends of the detection coil. Therefore, with this position transducer, the effective length for detection can be assured efficiently, and the length of the detection coil in relation to the effective length for detection is relatively decreased to design the position transducer compact on the whole and permit a high accuracy of the position detection.
Also, with the position transducer, the effective length for detection can be assured efficiently with simple measures such as a lap-winding of auxiliary coils on the end portions of the detection coil, so the position transducer can be designed compact on the whole without increase of the manufacturing costs.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.